Merck & Co. is making a record bet that an experimental disease treatment is worth more than Coppertone and Dr. Scholl's.

The drugmaker has offered to pay $3.85 billion for Idenix Pharmaceuticals Inc., a developer of hepatitis C drugs. The offer of $24.50 per share is almost quadruple the stock's average price during the past few weeks, the highest premium on record for any health care deal of at least $100 million.

With the Idenix deal, Merck is doubling down on the potentially $20 billion market for hepatitis C treatments after agreeing last month to sell its consumer division to Bayer AG. The moves signal that the $169 billion company is increasing its focus on riskier businesses that have the potential to yield bigger returns.

"If you've got a $20 billion industry and you can pay $4 billion to get into it, I think that's great," said Scott Redmond, founder of Redmond Asset Management.

"It's a huge market and for the next 10 years or so, there are going to be very, very few competitors whereas in the consumer products, there are competitors all over the place."

Redmond said his firm, which oversees about $235 million, sold its shares of Idenix after the takeover was announced.

The deal marks Merck's first acquisition of size since it bought Schering-Plough Corp. for more than $40 billion in 2009.

Idenix's lead drug, IDX21437, works by stopping the hepatitis C virus from replicating within the body. Combining that with Merck's existing regimen could improve potency and reduce treatment duration, Brian Abrahams, an analyst at Wells Fargo & Co., wrote in a report this week.

That will better position Merck to compete against rivals such as Gilead Sciences Inc. in the race to develop a more effective treatment for the estimated 170 million hepatitis C patients worldwide. Gilead and Merck plan to create drug combinations in the next few years that promise to work against most forms of the disease, and take less time to do it than current therapies.

Gilead of Foster City received Food and Drug Administration approval in December for Sovaldi, the $1,000-a-pill treatment it acquired in its $11 billion purchase of Pharmasset Inc. in 2012. The deal represented about a 90 percent premium for Pharmasset shareholders.

The 290 percent premium Merck is paying for Idenix is more than double the previous record set by Bristol-Myers Squibb Co. in its 2012 acquisition of Inhibitex Inc., another hepatitis C drug developer. Bristol-Myers had to take a $1.8 billion charge and discontinue development of the drug after a patient experienced heart failure.